1. Field of the Invention
This invention relates, in general, to optical printheads and, more specifically, to LED printheads for use in copiers, duplicators, and printers.
2. Description of the Prior Art
Optical printheads are used in copiers duplicators, and printers to expose a photoconductive surface or film in the apparatus in such a manner that a latent image is formed on the film. The image is later developed and transferred to paper for producing a hard copy output from the apparatus. Normally, optical printheads use light emitting diodes (LED's) to generate or produce the radiation necessary to expose the photoconductive film. In conventional printheads, the LED's are arranged in a linear array of LED's having a designed density to provide a resolution of a predetermined number of dots per inch. In other words, the greater the number of dots per inch desired to be printed, the greater will be the number of LED's grouped together in a linear string. In high resolution printheads, the requirements for the spacing between the LED's becomes critical.
In most cases, the LED's are provided on separate chip assemblies with each chip having several LED's, such as 128 per chip. Printheads having several thousand LED's in a linear array, therefore, require many chips to construct such an array. Since any spacing between the chips which is greater than the spacing between the individual LED segments on each chip will produce undesirable prints or copies, it has been desirable, according to the prior art, to mount the chips as closely as possible to each other. With lower resolution systems, this has not become a major problem. However, with the desire to go to higher resolution printing, and thus more closely spaced LED's, the spacing in the printhead between the LED chips is of critical significance. Not only is it a mechanical problem in spacing the LED chips, it becomes a problem of thermal expansion since printheads can develop a considerable amount of heat. Thus, irregardless of the ability to position the LED chips close together because of the structure of the chips, unless some means for compensating for the expansion of the printhead due to changes in temperature are present, a satisfactory printhead cannot be obtained for high resolution printing.
Thermal expansion of the printhead elements also can cause mechanical failure between the bonds of various members and surfaces within the printhead. In order to prevent this type of failure, it is necessary to allow for the difference in the thermal coefficient of expansion of the various members and materials used to construct the printhead. Therefore, it is desirable, and it is an object of this invention, to provide an optical printhead which can have the LED's arranged for high density printing and which can compensate for or tolerate materials in the construction of the printhead having significantly different coefficients of thermal expansion.